1. Field of the Invention
This invention relates generally to the art of studying the physical properties of particles carried in suspension and more particularly is concerned with improved apparatus for obtaining signals from particles passing through a scanning aperture without extraneous interference from other particles.
2. Description of the Prior Art
Particles of microscopic size and larger are counted and sized throughout the world by use of an apparatus known as the Coulter Counter.RTM.. These apparatuses are constructed in accordance with the principles of U.S. Pat. No. 2,656,508 to Coulter, wherein there are a pair of vessels, which are formed of insulating material and have a minute aperture between them. A suspension of the particles to be studied flows from one vessel to the other through the aperture. An electrical current is established through the aperture by use of suitable electrodes suspended in the respective bodies of electrolyte liquid in the vessels and an electrical power source is connected across the electrodes. The displacement of fluid within the sensing zone of the apparatus, which zone includes the aperture, by the presence of a particle therein will cause a change in the impedance of the sensing zone, and this change is detected by suitable detecting means connected to the electrodes.
An apparatus disclosed in U.S. Pat. No. 3,299,354 to Hogg substantially decreases the possibility of undesirable spurious particle reading and count signals which sometimes occured in prior art devices, such as that disclosed in U.S. Pat. No. 2,656,508. This is accomplished by replacing the so-called aperture tube of prior art structures with a pair of chambers having an interconnection for separating the electrical and mechanical effects produced by particles passing through the aperture. Particles passing through the aperture of the apparatus immediately are transported away from the proximity of the aperture so that there is little or no chance of spurious signals resulting from said particles.
One of the objects of the invention disclosed in U.S. Pat. No. 3,299,354 was to provide an aperture tube which was self-cleaning in that the suspension in the immediate vicinity of the aperture was kept free of extraneous particles. However, eddy currents of fluid in the aperture tube on the downstream side of the aperture could occur and these eddy currents swirled into the sensing zone of the aperture. It was believed that the fluid downstream of the aperture would be stagnant liquid substantially devoid of particles, hence not introducing appreciable extraneous signals. While this was largely true, the action has been determined not to be sufficient to satisfy the more critical demands of today's technology. A small percentage of particles was not caught by the orifice in the elongate neck of the second chamber and these sometimes produced extraneous signals by virture of the eddy currents at the bottom of the first chamber of the aperture tube.
For purposes of resolving the aforesaid problems with regards to extraneous signals, the invention disclosed in U.S. Pat. No. 3,746,976 to Hogg and U.S. Pat. No. Re. 28,558 to Hogg provides a self-cleaning aperture tube with the addition of a pump device interposed between the first and second chambers to produce a closed system in which there are no inlets or outlets other than the aperture in the first chamber and a waste outlet. The pump operates to draw the particle suspension up through the second chamber and force the same back into the first chamber, completing a circuit around this path and creating a sheath flow in the first chamber to hydrodynamically focus the particles as they proceed toward an orifice into the second chamber. The flow created by the pump is such as to ensure that all particles introduced into the aperture tube are passed through, i.e. caught by the orifice of the second chamber so as to prevent the occurrence of extraneous signals.
The invention of U.S. Pat. No. 3,746,976 satisfactorily eliminates the extraneous signal problems inherent in the apparatus of U.S. Pat. No. 3,299,354. In so doing, however, the structure incorporates a pump and filter through which electrolyte is recirculated to achieve the cleaning flow. Further, the structure of U.S. Pat. No. 3,746,976 includes a relatively fragile and cumbersome aperture tube in which the chambers thereof are separated along a substantial portion of the tube.
The apparatus disclosed in U.S. Pat. No. 3,902,115 to Hogg, et al. provides an improved self-cleaning aperture tube. The aperture tube eliminates the above described disadvantages of the tube disclosed in U.S. Pat. No. 3,746,976 in which one chamber has a branch of lateral diversion which passes out of a second chamber and returns again. Further, the apparatus provides a system to supply fresh clean electrolyte from a reservoir and eliminates the need for recirculating the electrolyte through a pump and filter as in the structure of U.S. Pat. No. 3,746,976.
The apparatus of U.S. Pat. No. 3,902,115 uses an elaborate, non-circulating reservoir system, which requires the wasteful use of large quantities of electrolyte for the liquid sheath.
The above mentioned patents, U.S. Pat. Nos. 2,656,508; 3,299,354; 3,746,976; U.S. Pat. Nos. Re. 28,558; and 3,902,115 are incorporated herein as a part hereof by specific reference to the extent that incorporation may be needed for a better appreciation of the invention herein.